Since lithium-ion secondary batteries have a high energy density, they have been used widely as a power source for portable equipment. However, an electrolyte in the conventional lithium-ion secondary batteries is prepared by dissolving an electrolyte salt such as an alkali metal salt into a carbonate-type solvent with a low boiling point. Because of this low boiling point of the carbonate-type solvent used as the solvent for the electrolyte, the electrolyte has a high vapor pressure, which tends to cause battery swelling during storage and dangerous leakage, posing problems in safety and storage characteristics.
Accordingly, for the purpose of lowering the vapor pressure, JP 2(1990)-34660 A, for example, suggests that a mixture of a crosslinked polymer having a structural unit of a random copolymer of ethylene oxide and propylene oxide, an uncrosslinked random copolymer having a structural unit similar to the above and an alkali electrolyte salt should be used as the electrolyte.
Although the above-mentioned electrolyte using the polymer based on ethylene oxide and propylene oxide has a high boiling point and a low vapor pressure and is effective in improving the battery safety, there still has been a problem of low lithium-ion transport number. For example, as to ether oxygen such as ethylene oxide and propylene oxide, 4 to 6 ether oxygen atoms coordinate with one lithium ion so as to dissociate a lithium electrolyte salt. Since such a lithium ion coordinated by ether oxygen atoms is prevented from moving to a greater degree compared with a counter anion coordinated by no ether oxygen atoms, the lithium-ion transport number lowers. In other words, even though the apparent ionic conductivity is high, the lithium-ion transport number is less than 0.2 at maximum and lower than the lithium-ion transport number of a carbonate-type solvent (0.2 to 0.3).
In the case where the electrolyte has a low lithium-ion transport number as described above, polarization tends to occur. Thus, when the electrolyte is used in a battery, an IR drop increases, leading to a problem that a large electric current cannot be generated.
Accordingly, as a more effective alternative for improving the safety, the use of a polymer electrolyte having an ionic conductivity is considered. Since this polymer electrolyte does not have a vapor pressure in a working temperature range of a battery and can be formed into a sheet, it becomes possible to produce a thin battery having a large area such as an A4 size or a B5 size. A polymer electrolyte battery using this electrolyte can be made into a thin and flexible battery having excellent safety and storage characteristics including leakage resistance, so that it has characteristics exceeding other batteries in that a battery conforming to the shape of equipment can be designed. Thus, the usable range of the battery can be expanded greatly; for example, the battery becomes applicable to various thin electric appliances.
The above-described polymer electrolyte usually is formed by mixing an electrolyte salt of an alkali metal uniformly into a base polymer. Conventionally, electrolytes using various polymers as this base polymer have been suggested. For example, JP 61(1986)-83249 A suggests a battery using a polymer electrolyte composition obtained by dissolving a lithium salt into polyether such as polyethylene oxide or polypropylene oxide or compositions thereof.
Further, as the polymer electrolyte, a polymer electrolyte and a gel electrolyte using a polymer having a carbonate group in its molecule are suggested in JP 62(1987)-30147 A, for example.
However, these polymer electrolytes have an extremely low ionic conductivity (σ=10−5 S/cm or lower) around room temperature and fall short of the practical level. This is because a high crystallinity of the polymers used therein limits the molecule motion around room temperature, thus lowering the mobility of ions serving as a dominant factor of ion conduction. Therefore, batteries using these polymer electrolytes can be charged and discharged only at a high temperature of 60° C. or more and used in a greatly limited range at present.
In order to solve the above problem, various attempts have been made to lower the crystallinity of polyether, which is a typical base polymer of a polymer electrolyte. For example, Macromolecules, 32, (1999), p. 1541 reports that, for lowering the crystallinity of an ethylene oxide chain, an improvement of crosslinking or greatly branching polyether can achieve an ionic conductivity on the order of 10−4 S/cm.
However, such attempts have not yet overcome the intrinsic disadvantage of a low lithium-ion transport number of the polymer electrolyte using polyether. In other words, the lithium-ion transport number is lower than 0.2 at the highest and lower than the lithium-ion transport number of a liquid ion-conducting electrolyte (0.2 to 0.3).
As described above, an ion-conducting electrolyte having a low vapor pressure, a high ionic conductivity even at room temperature and a high lithium-ion transport number has not yet been developed. Accordingly, a battery having a high level of safety and excellent discharge characteristics has not yet been achieved.